(a) Field of the Invention
The present invention relates to an acoustic resonator and a filter. In particular, it relates to a thin film bulk acoustic resonator and a filter for use in an electronic circuit.
(b) Description of Related Art
Aiming at improvement in performance and reduction in cost and size of electronic devices, there are growing demands for high frequency signal filters of small size and high performance. On electronic products for consumer use that make use of high frequency signals, such as cellular phones, severe limitations have been imposed in terms of cost. Further, since a signal of a certain frequency must be selected accurately among input high-frequency signals, precise filters have been required. In such conditions, research and development have persistently been carried out to realize an inexpensive and simple filter unit.
Nowadays, attention has been paid to a filter using a thin film bulk acoustic resonator (FBAR) as an example of possible filters that meet the above-described requirements. The FBAR is an acoustic resonator which makes use of bulk longitudinal acoustic waves in a thin piezoelectric (PZ) film sandwiched between two metal electrodes.
The PZ film sandwiched between the electrodes is a resonator film which is preferably not fixed but suspended in the air. By so doing, when an elastic wave generated in the PZ film by converting electrical energy to mechanical vibration is confined in the PZ film, the elastic wave is propagated in the direction of an electric field and reflected at the interface between the electrodes and the air, thereby preventing attenuation of the mechanical vibration.
To obtain the resonator film suspended in the air, a through hole or a depression is formed in a substrate which supports the resonator film, or a depression is formed in a support layer which is formed between the substrate and the resonator film.
For example, the specification of U.S. Pat. No. 6,384,697 discloses a resonator including a bottom electrode, a PZ film and a top electrode formed in this order on a substrate to cover a cavity formed in the substrate. The specification also discloses a method of fabricating a resonator having a cavity by forming the cavity by etching a depression into the substrate, forming a sacrificial layer made of SiO2, forming a bottom electrode, a PZ film and a top electrode in this order on the sacrificial layer formed on the substrate, forming an opening and removing the sacrificial layer via the opening by etching.
Further, Japanese Unexamined Patent Publication No. 2002-509644 (corresponding to International Application WO98/16956) discloses a method of fabricating FBARs by forming an insulating protective film, a bottom electrode, a PZ film and a top electrode on a substrate in this order so that a via hole formed in the substrate is covered. The Publication further discloses the steps of forming a sacrificial layer on a selected portion of the substrate, forming an insulating protective film, a bottom electrode, a PZ film and a top electrode in this order on the sacrificial layer, forming a via hole in the selected portion of the substrate and removing the sacrificial layer through the via hole by etching.
Japanese Unexamined Patent Publication No. H9-64675 discloses a resonator including a bottom electrode, a PZ film and a top electrode formed on a SiO2 layer which is formed on a substrate to cover a cavity formed in the substrate. According to the Publication, the resonator is formed by the steps of forming a cavity in the substrate by etching, bonding the SiO2 layer to the substrate in which the cavity has been formed and stacking the bottom electrode, PZ film and top electrode in this order on the SiO2 layer.
In the thus configured resonator, the cavity is provided below a resonator film including the bottom electrode, the PZ film and the top electrode. Therefore, an elastic wave generated in the resonator film is confined in the resonator film with efficiency.
In a resonator of a conventional structure, however, the cavity exists only below part of the resonator film where electrical resonance occurs and a support member for supporting the resonator film is provided below other parts of the resonator film, thereby hindering vibration of the resonator film.
To meet the requirements for the filters such as broadband and low loss, it is necessary to increase the difference (Δf) between resonant frequency and antiresonant frequency of the resonator of the FBAR and to raise a selectivity value (Q). To obtain a large Δf, the resonator film needs to vibrate without restraint. When the resonator film is suspended in the air in the absence of the support, the value Δf will be the largest. However, since a certain structure for supporting the resonator film is indispensable in reality, it is preferable that the resonator film vibrates as freely as possible.
A filter is formed of two or more FBARs arranged adjacent to each other. Therefore, if the adjacent FBARs are supported by the supports contacting each other, the vibration of one of the FBARs is transmitted to the other via the contacting supports, thereby causing unwanted resonance (spurious vibration) and a decrease in Δf.